RE: [APML] Satellite avoidance in photos

["Jon Kolb" <jkolb@datawest.net>]

Roland,

> Sure, that's why I would want to restrict it to a given time 
> interval.  If I'm interested in, say satellites brighter than 
> mag 4 passing through Hercules between 11:00 PM and midnight 
> on June 14th (just happens to be what I *am* interested in), 
> that number is much smaller.  It appears to be about 3.

Well sure, if you can limit your search to a small patch of sky like
that, and filter on magnitude as well, then the number will be much
smaller.  Your original message didn't make that clear, or I missed that
point if it did.

I don't recall if you said you were using The Sky to do this, but you
should make sure you have updated element sets with epoch times as close
as possible to the time you need the information, or whatever prediction
it's giving you is going to be inaccurate.  The 2-card (nowadays called
two-line) element sets available on-line from NORAD are typically
corrected three times per day (in the NORAD system) for near-earth
orbits because they change rather quickly.  In addition, the general
perturbations propagation models available on-line and in many software
packages are not accurate for long term predictions, so current elements
are required for accuracy.  It's also possible that the software you
have is not entirely compatible with a two-line element set (a topic
that's a bit more complicated), but that's probably less likely.

I'm not sure how often the various web sources for element sets are
updated, but it's probably more like once per day and this should
suffice for your purposes.  I believe that Heavens Above uses pretty
current NORAD elements.  

> In any event, having people tell me that I really only need 
> to worry about the really bright ones, and that most of the 
> satellites I can see won't show up on my shots.  That makes 
> sense, especially since I haven't seen any in my past shots 
> (uhm, all 4 of them) even though I took no precautions to avoid them.

Yeah, and chances are that you won't pick up many on the next 4, or 40
either.  In over five years of serious astrophotography, I can only
remember having to remove a couple that otherwise spoiled a shot.
Sometimes I see very faint trails, but they're so innocuous that nobody
can see them without a magnifier on an 11x14 print.  Unless it's an
Iridium flare or an ISS pass right through your field, I wouldn't spend
much effort trying to plan them out of your shots.   You will find that
much greater frustration can be had from airplanes.

Sincerely,
Jon Kolb
Adventures in Astrophotography
http://home.datawest.net/jkolb/
jkolb@datawest.net




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